linux-kernel - Re: [PATCH] mm: workingset: ignore slab memory size when calculating shadows pressure

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Date:   Thu, 3 Sep 2020 22:02:47 -0700
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm: workingset: ignore slab memory size when calculating
 shadows pressure

On Thu, Sep 03, 2020 at 09:10:59PM -0700, Andrew Morton wrote:
> On Thu, 3 Sep 2020 16:00:55 -0700 Roman Gushchin <guro@...com> wrote:
> 
> > In the memcg case count_shadow_nodes() sums the number of pages in lru
> > lists and the amount of slab memory (reclaimable and non-reclaimable)
> > as a baseline for the allowed number of shadow entries.
> > 
> > It seems to be a good analogy for the !memcg case, where
> > node_present_pages() is used. However, it's not quite true, as there
> > two problems:
> > 
> > 1) Due to slab reparenting introduced by commit fb2f2b0adb98 ("mm:
> > memcg/slab: reparent memcg kmem_caches on cgroup removal") local
> > per-lruvec slab counters might be inaccurate on non-leaf levels.
> > It's the only place where local slab counters are used.
> > 
> > 2) Shadow nodes by themselves are backed by slabs. So there is a loop
> > dependency: the more shadow entries are there, the less pressure the
> > kernel applies to reclaim them.
> > 
> > Fortunately, there is a simple way to solve both problems: slab
> > counters shouldn't be taken into the account by count_shadow_nodes().
> > 
> > ...
> >
> > --- a/mm/workingset.c
> > +++ b/mm/workingset.c
> > @@ -495,10 +495,6 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker,
> >  		for (pages = 0, i = 0; i < NR_LRU_LISTS; i++)
> >  			pages += lruvec_page_state_local(lruvec,
> >  							 NR_LRU_BASE + i);
> > -		pages += lruvec_page_state_local(
> > -			lruvec, NR_SLAB_RECLAIMABLE_B) >> PAGE_SHIFT;
> > -		pages += lruvec_page_state_local(
> > -			lruvec, NR_SLAB_UNRECLAIMABLE_B) >> PAGE_SHIFT;
> >  	} else
> >  #endif
> >  		pages = node_present_pages(sc->nid);
> 
> Did this have any observable runtime effects?

Most likely not.

I maybe saw the second effect once, but it was backed up by a bug in the inode
reclaim path in the exact kernel version I used (not an upstream one).

The first problem is pure theoretical, I'm just not comfortable with using these
counters, which are known to be inaccurate after reparenting.

That's why I didn't add stable@.

Thanks!